We have demonstrated that the "masked" mRNA of unfertilized sea urchin eggs is present in the form of ribonucleoprotein particles, and that the post-fertilization rise in protein synthesis results from both mRNA availability and translational efficiency. We are investigating the proteins of egg mRNPs and are comparing nontranslatable egg particles with the translatable mRNPs of embryo polysomes. We are further examining the flow of specific stored mRNA species into polysomes in embryos, and modeling the control of protein synthesis based upon measurable parameters of mRNA recruitment and translational efficiencies of the translation machinery.